Substantially all offshore oil and gas production is conducted from rigid concrete or steel structures fixedly secured to the ocean bottom and extending upward to a work deck above the ocean surface. These structures are provided with sufficient rigidity and foundation strength to resist waves, ocean currents and wind without significant motion. For relatively shallow depths, these conventional rigid structures have proven a reliable and economic means for tapping marine hydrocarbon reserves. However, in recent years, the search for offshore oil has extended into water depths in excess of 300-400 meters. At these depths, providing a production structure with sufficient rigidity and foundation strength to resist motion under the action of environmental forces requires a massive, often prohibitively expensive design. Because of this, much recent work has been performed to develop drilling and production structures which avoid the depth sensitivities inherent to conventional rigid structures.
One of the most promising concepts for structures useful in deep water offshore areas is the tension leg platform. Tension leg platforms are designed to have a compliant rather than rigid response to environmental forces. Under the action of waves, wind, and ocean currents, a tension leg platform undergoes limited, long period motion. By avoiding a need for structural rigidity, significantly less structural material is required than would be necessary for a conventional rigid structure, resulting in decreased cost.
A typical tension leg platform is illustrated in FIG. 1. Tension leg platforms have a buoyant main body (the "hull") which floats at the ocean surface and supports the drilling rigs and other equipment used in drilling and production activities. The hull is secured to a foundation on the ocean floor by a set of tethers. The length of the tethers from the ocean floor to the hull is carefully adjusted to ensure that the hull is maintained at a somewhat greater draft than would be the case were the hull unrestrained. The resulting buoyant force of the hull exerts an upward load on the tethers, maintaining them in tension. The tensioned tethers substantially restrain the hull from pitch, roll and heave motions induced by waves, ocean currents and wind. By relying on a tensile rather than compressive loading of the structure securing the hull to the ocean floor, the depth sensitivities inherent to conventional structures are largely avoided. It has been suggested that tension leg platforms could be employed in depths up to 3000 meters (9840 feet), whereas the deepest present application of a conventional rigid structure is in a water depth of approximately 412 meters (1350 feet).
Though tension leg platforms avoid many of the disadvantages inherent to conventional rigid structures in deep water, they do present several unique design problems. One of these has centered on the development of a connector which will allow the tether to be secured to and removed from the ocean floor foundation. Because of the great water depths at the installation location, the connector must be remotely operable. The connector must also be adapted to permit the tethers to be repeatedly disconnected and reattached over the life of the structure for tether inspection and maintenance. Because the location of the connector makes maintenance difficult, it should also be mechanically simple. Further, the great length and mass of the tether greatly complicates manipulation of the tether, placing a premium on simplicity of connector operation. Thus, a stab-type connector is generally preferred over, for example, a threaded connector. It is also necessary, of course, that the connector be rugged and of sufficient strength to support the considerable loads which must be transferred from the hull to the foundation.
One concept for a tension leg platform tether connector is set forth in U.S. Pat. No. 4,459,933, issued July 17, 1984. This connector uses a hydraulically operated collar to push spacer blocks between a load ring on the connector element at the base of the tether and a load ring on a tether receiving chamber secured to the ocean floor foundation. The tether is released by retracting the collar. Though this connector is effective, its reliance on hydraulic power presents a significant disadvantage. Failure of the hydraulic operator would delay, and could prevent, removal of the tether. Additionally, the need to transmit hydraulic power from the hull to the tether bottom complicates tether design and handling.
It would be desirable to develop a connector which would permit a tension leg platform tether to be remotely and easily secured to and removed from a foundation template at the ocean bottom without the need for powered actuators.